Growth of thick highly boron-doped diamond single crystals by CVD A. Tallaire, R. Issaoui, F. Silva, V. Mille, J. Achard, A. Gicquel LIMHP-CNRS, Université Paris XIII, 99 avenue JB Clément, 93430 Villetaneuse, FRANCE Keywords : CVD diamond, crystal growth, boron doping, power electronic devices ABSTRACT The fabrication of vertical diamond-based power-electronic devices is expected to allow overcoming the high series resistance that still hampers the development of traditional coplanar devices. However this requires that thick (> 100 µm) heavily-doped diamond crystals with a large usable top surface are grown. With this objective in mind, high microwave power densities are necessary so that reasonable growth rates and high crystal qualities are reached. In this paper we studied the impact of the microwave power density coupled to the plasma and the ([B]/[C])gas ratio on the growth of thick boron-doped diamond layers. It was found that the doping efficiency as measured by SIMS drastically drops by almost 2 orders of magnitude at high microwave power densities. Besides we used a 3D geometrical model previously developed at LIMHP in order to predict the final crystal morphology. It was found that the addition of boron to the gas phase promotes the appearance of large {110} and {113} crystalline faces on the edges and corners of the crystal. The optimal growth conditions in terms of defect formation, crystal morphology and boron doping are discussed here and the first thick highly-doped diamond layers are reported. INTRODUCTION Diamond is a very promising material to fabricate power-electronic devices. Although n-type doping is still relatively difficult to obtain due to the lack of an efficient donor, p-type doping can now be reliably achieved using boron as an acceptor impurity, thus potentially opening the way to the fabrication of unipolar devices. The first diamond-based coplanar structures fabricated have shown encouraging results1. However when very high currents (> 100 A) are required, the devices suffer from a high on-state resistance. Using a vertical configuration in which Ohmic and Schottky contacts are deposited on each side of the diamond crystal is a possible way reduce this parameter. This has been supported by recent modelling results2. Therefore, our aim is to develop a process that allows the fabrication of thick (> 100 µm) boron-doped diamond layers that could be used as a platform for power electronic devices. These layers should be sufficiently thick to be mechanically robust and detached from the substrate to provide a freestanding plate. Moreover, to reduce the contact and access resistance, they should be highly doped and ideally, metallic transition should be reached. The quality of the layers is also crucial since in such highly demanding applications, the presence of non-epitaxial crystallites or dislocations have a deleterious effect on the electronic properties. Finally the top surface onto which the contacts are deposited should be as large possible. All these are stringent requirements for the synthesized material that need working on the PACVD deposition conditions and improving the process. In this paper the growth conditions required to synthesize thick heavily boron-doped diamond single crystals by plasma assisted CVD are discussed. The influence of the ([B]/[C])gas ratio as well as the microwave power density (MWPD) will be presented especially regarding the doping level and quality of the layer. Besides the final morphology of the diamond crystal is of prime importance and will be reported on the basis of results obtained using a 3D geometrical model.

EXPERIMENTAL For the growth of boron-doped layers, high-pressure high-temperature (HPHT) Ib (001)-oriented single crystal diamonds have been used as substrates. The diamond layers were grown by microwave plasma assisted CVD in a bell jar reactor commercialized by the company Plassys. Boron doping was achieved using high purity methane (CH4) at a concentration of 5% and diborane (B2H6) diluted in H2. For all experiments, the substrate temperature was maintained at 880°C. The MWPD was varied by changing both the pressure and the input microwave power. Boron concentration in the layer was measured by SIMS and the thickness was deduced from the weight and thickness difference before and after deposition. Modelling of the crystal morphology was carried out using a 3D geometrical model which has been presented elsewhere3. The displacement rates of the {100}, {110}, {111} and {113} faces were experimentally measured by growing diamond layers on different substrate orientations. These values were then injected into the model in order to predict the final morphology for the growth of 500 µm-thick films. RESULTS AND DISCUSSION Fig. 1a shows the boron concentration measured by SIMS for films grown with different MWPD and using two ([B]/[C])gas ratios (500 ppm and 5,000 ppm). The corresponding doping efficiency (DE) which represents the ratio between the boron concentration in the solid to that in the gas phase (([B]/[C])gas/([B]/[C])sol) is also plotted in Fig. 1b. These results show that the DE drastically drops when the MWPD is increased. A possible explanation for this behaviour is a change in plasma chemistry. The gas temperature increases with the MWPD which can lead to an enhanced BHx dissociation. Therefore it is possible that the precursor species involved in boron doping are consumed in the bulk of the plasma at high MWPD thus leading to a reduced DE. 1D axial plasma optical study and modelling are in progress in our laboratory to correlate this experimental observation with plasma species density. 2.0

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(a) (b) Fig. 1. (a) Evolution of boron concentration measured by SIMS as a function of microwave power density (MWPD) (b) Corresponding doping efficiency (DE) as a function of microwave power density

Another issue at high MWPD comes from the very high diborane concentrations that are required to dope the material. This is responsible for soot formation and plasma unstability. The growth cannot be sustained for a long period of time. To the contrary too low a MWPD comes with a substantial morphology degradation and a trade-off in the achieved growth rates. As a consequence it was found that a medium MWPD has to be used in order to ensure that thick highly boron-doped layers can be obtained within a reasonable deposition time. Fig. 2a shows for example a thick highly-doped freestanding diamond plate that was obtained using a MWPD of 60 W/cm3.

Regarding the morphologies of boron doped diamond crystals, the results obtained using the 3D geometrical model are presented in Fig. 3 for different ([B]/[C])gas ratios. It was observed that the introduction of boron impurities to the gas phase results in a decrease of the growth rates of {110} and {113} faces. As a consequence the crystal morphology shows large {113} and {110} faces at the corners and edges respectively. This was confirmed when growing a 250 µm-thick diamond layer shown in Fig. 2b where large {110} crystalline faces can be seen. One issue related to these faces comes from the stress that they induce in the crystal. This can lead to its breaking up as can be seen in the 4 corners of the crystal in Fig. 2b and as was reported elsewhere4.

(a) (b) Fig. 2. (a) Optical image of a freestanding boron doped diamond layer (Thickness of 250 µm after polishing), (b) SEM image of a 250 µm-thick as-grown diamond crystal obtained with a [B]/[C] gas of 800 ppm and showing large {110} faces and {113} broken corners.

(b) (c) (a) (d) Fig. 3. Crystal morphology predicted by the 3D geometrical model after a 500 µm-thick CVD diamond layer was grown. {110} faces are visible at the edges and {113} faces at the corners. (a) [B]/[C]gas = 60 ppm, (b) [B]/[C] gas = 800 ppm, (c) [B]/[C] gas = 2,000 ppm, (d) [B]/[C] gas = 5,000 ppm

In summary, the effect of the MWPD and boron concentration on the crystal morphology of thick CVD diamond layers was studied. We found that only a very narrow window of deposition conditions allows obtaining thick heavily boron-doped diamond layers with a reasonable growth rate and good surface morphologies. In particular, it is crucial to use medium MWPD (50-70 W/cm3). Boron has been shown to promote the appearance of large {113} and {110} crystalline faces. These latter induced stress in the crystal and led to its breaking-up. In spite of this, we have successfully managed to grow thick freestanding highly-doped diamond layers. They could be the first building block leading to the fabrication of vertical diamond-based power devices. ACKNOWLEDGEMENTS This work was supported by the French DGE project Diamonix. REFERENCES 1

Y. Garino, T. Teraji, S. Koizumi, Y. Koide, T. Ito, Phys. Stat. Sol. (a) 206 (2009) 2082 J. Achard, F. Silva, R. Issaoui, O. Brinza, A. Tallaire, H. Schneider, K. Isoird, H. Ding, S. Kone, M. A. Pinault, F. Jomard, A. Gicquel, Submitted to Diam. Relat. Mater. (2010). 3 F. Silva, X. Bonnin, J. Achard, O. Brinza, A. Michau, A. Gicquel, Journal of Crystal Growth 310 (2008) 187. 4 O. Brinza, J. Achard, F. Silva, X. Bonnin, P. Barroy, K. De Corte, A. Gicquel, Phys. Stat. Sol. (a) 205 (2008) 2114 2